One of Two Raster Surfaces Does Not Show in Spatial Analyst Drop Down List

I have a map with two raster surfaces. In the drop down list for Spatial Analyst only one surface appears for selection. I can work with that one surface (adding contour lines, etc.) but not the other. I can find no clue in Help or online documentation as to why this is happening.

I confess ignorance. I am not familiar with the different surface formats, and I am not in the office so I can't look at the two surface properties. I do know that I constructed the one surface with Spatial Analyst by interpolation from a table of lat-long and elevation. That surface works fine with Spatial Analyst. The other surface I pulled from a .mdb file that was given to me. That surface displays fine on the map, but I can't get it to work with Spatial Analyst.

You're best bet may be to right click on the layer in the right hand-side Table of Contents and select Data>Export Data, then save the raster surface as a grid. It normally solves most raster surface errors.